1. Field of the Invention
The present invention relates to a laser beam scanning optical apparatus, and more particularly to a laser beam scanning optical apparatus which is employed in a laser printer, a digital copying machine or the like as printing means.
2. Description of Related Art
Recently, a laser beam scanning optical apparatus which is employed in a laser printer, a digital copying machine or the like as printing means is structured to be capable of printing with a high density for improvement of picture quality. Accordingly, the beam spot on a scanning surface (photosensitive member) must be small, and the focal depth is small. When the environments change, especially when the optical apparatus heats, thereby causing thermal expansion of the optical elements and/or a holder thereof, the image point shifts and comes before or after the scanning surface. The shift of the image point is not allowed in order to keep picture quality.
In order to solve the problem, Japanese Patent Laid Open Publication No. 2-58016 discloses that a focus detecting device is provided in a position which is optically equivalent to the scanning surface to detect the focusing of the laser beam and that a focusing lens is moved to a proper position according to the result of the detection. Recently, by request for high-speed printing, image forming apparatuses have a very high scanning speed. Accordingly, the conventional focus detecting device requires a high-speed responsive detection circuit, and it is necessary to use an element which can sample the output of a light receiving element at such short intervals as MHz or GHz, thereby raising the cost.